In many electronic applications, electrical resonators are used. For example, in many wireless communications devices, radio frequency (RF) and microwave frequency resonators are used as filters to improve reception and transmission of signals. Filters have traditionally included inductors and capacitors, and more recently include acoustic resonators.
As will be appreciated, it is desirable to reduce the size of many components of electronic devices. Certain known acoustic filter technologies present a barrier to overall system miniaturization. With the need to reduce component size, a class of resonators based on the piezoelectric effect has emerged. In piezoelectric-based resonators, acoustic resonant modes are generated in the piezoelectric material. These acoustic waves can be converted into electrical waves for use in electrical applications.
One type of piezoelectric resonator is a Bulk Acoustic Wave (BAW) resonator. The BAW resonator includes an acoustic stack comprising, inter alia, a layer of piezoelectric material disposed between two electrodes. Acoustic waves achieve resonance across the acoustic stack, with the resonant frequency of the waves being determined by the materials of the layers of the acoustic stack, and the thicknesses of the layers of the acoustic stack. One particular type of BAW resonator comprises a thin film piezoelectric layer for the piezoelectric material. These resonators are often referred to as Film Bulk Acoustic Resonators (FBAR).
FBARs are similar in principle to bulk acoustic resonators such as quartz, but are scaled down in size to resonate at GHz frequencies. Because the FBARs have thicknesses on the order of micrometers (microns) and length and width dimensions of hundreds of microns, FBARs beneficially provide a comparatively compact alternative to certain known resonators.
FBARs may comprise an acoustic stack disposed over air. In such a structure, the acoustic stack is sometimes referred to as a membrane. Typically, the membrane is suspended over a cavity provided in a substrate. In other BAW resonator structures the acoustic stack is disposed over an acoustic mirror formed in the substrate. Regardless of whether the acoustic stack is suspended over air or provided over an acoustic mirror, the acoustic stack comprises a piezoelectric layer disposed over a first electrode, and a second electrode disposed over the piezoelectric layer.
Filters based on FBAR technology provide a comparatively low in-band insertion loss due to the comparatively high quality (Q) factor of FEAR resonators. FBAR-based filters are often employed in cellular or mobile telephones that can operate in multiple frequency bands. In such devices, it is important that a filter intended to pass one particular frequency band (“the passband”) should have a high level of attenuation at other nearby frequency bands, which contain signals that are desirably rejected. Specifically, there may be one or more frequencies or frequency bands near the passband which contain signals at relatively high amplitudes that should be rejected by the filter. In such cases, it would be beneficial to be able to increase the filter's rejection characteristics at those particular frequencies or frequency bands, even if the rejection at other frequencies or frequency bands does not receive the same level of rejection.
One type of filter based on FBAR technology is known as a coupled resonator filter (CRF). A CRF comprises a coupling structure disposed between two vertically stacked FBARs. The CRF combines the acoustic action of the two FBARs, which leads to a bandpass filter transfer function. For a given acoustic stack, the CRF has two fundamental resonance modes, a symmetric mode and an asymmetric mode, of different frequencies. At any other frequency the CRF filter acoustic response is governed by a linear combination of the symmetric and asymmetric modes. The degree of difference in the frequencies of the modes depends, inter alia, on the degree or strength of the coupling between the two FBARs of the CRF. If the degree of coupling between the two FBARs is too great, the passband is unacceptably wide, and an unacceptable ‘swag’ or ‘dip’ in the center of the passband results. Moreover, if the degree of coupling between the two FBARs is too great, the insertion loss at the center of the passband is unacceptably high. Alternatively, if the degree of coupling is too weak for certain RF applications, the passband is unacceptably narrow.
The dependence of the passband on the degree of coupling has lead efforts to attempt to control the degree of coupling between the FBARs of the CRF. For many materials commonly used for acoustic applications at RF frequencies, the degree of coupling resulting from the interaction between the coupling material and the FBARs is too great, and results in an unacceptably high difference in the resonance frequencies of the modes of the CRF. Among other drawbacks, this results in an unacceptable ‘dip’ or ‘swag’ in the center of the passband, and unacceptable spreading of the passband.
One known technique used to control the degree of coupling between the FBARs of the CRF involves the use a coupling structure comprising a plurality of coupling layers with alternating high and low acoustic impedances. At each interface between each coupling layer a partial reflection of the acoustic mode occurs. The multiple interfaces provide a multiplicative reflective effect, and the degree of coupling between the FBARs can be beneficially controlled even when materials with relatively high acoustic impedances are employed in the coupling structure. While coupling structures comprising a plurality of coupling layers facilitate decoupling of the FBARs in the CRF, their presence adds complexity to the fabrication process, and ultimately to the cost of the resultant product.
What is needed, therefore, is a BAW resonator structure that overcomes at least the known shortcomings described above.